Nasa scientists have witnessed a rare explosion on the Moon, caused by a meteoroid slamming into it.

The blast was equal in energy to about 70kg of TNT and was seen near the edge of Mare Imbrium (the Sea of Rains).

The object that hit the Moon was probably part of a shower of "taurids" which peppered Earth in late October and early November.

Understanding lunar impacts could help protect astronauts when Nasa sends humans back to the Moon.

Meteoroids are small rocky or metallic objects in orbit around the Sun, or another star. One of the astronomers who observed the impact estimates that it gouged a crater 3m wide and 0.4m deep.

Rob Suggs of Nasa's Marshall Space Flight Center in Huntsville, US, was testing a new 10-in telescope and video camera assembled to monitor the Moon for space strikes.

On 7 November, his first night using the telescope, he observed one.

Renewed interest

"People just do not look at the Moon anymore," said Dr Suggs, of Marshall's engineering directorate.

"We tend to think of it as a known quantity; but there is knowledge still to be gained here."

Dr Suggs used commercial software to study the video he took, and spotted a very bright flash. The burst of light diminished gradually over the course of five video frames, each 1/30th of a second in duration.

The explosion can be seen in these video frames

He and Nasa astronomer Bill Cooke consulted star charts and lunar imaging software, and determined the meteoroid was probably a taurid, part of an annual meteor shower active at the time of the strike.

Like Earth, the Moon was peppered by taurids in late October and early November.

But unlike our planet, the Moon has no atmosphere to intercept and vaporise them, so they explode on the surface.

Since the Leonids of 2001, astronomers have not spent much time hunting for lunar impacts.

However, as Nasa plans to return to the Moon by 2020, the agency says it needs to understand what happens after lunar impacts in order to protect astronauts.

Dr Suggs said planetary scientists wanted to know how often big meteoroids hit the Moon and whether they only happened during showers like the taurids or were a more common occurrence.

Bill Cooke said that while the odds of a direct hit with a big meteoroid were almost nil for an individual astronaut, they might be shorter for an entire lunar outpost.